1. Field of the Invention
The present invention relates to the generation of acoustic waves from electronic signals to provide audible sounds, and more particularly, to a speaker apparatus which utilizes the interaction between a current carrying conductor and a magnetic field generated external to the speaker apparatus, to drive a speaker cone.
2. Description of the Prior Art
Modern music systems convert electronic signals from an audio amplifier to sounds using a conventional electrodynamic loudspeaker. While such loudspeakers are readily available, relatively inexpensive and operate well in most home and business environments, they are ineffective in environments in which large magnetic fields are present.
Conventional loudspeakers employ a permanent magnet and cone which are rigidly attached to a speaker frame. The cone typically has a voice coil attached, which together with the permanent magnet, make up the driver of the speaker. The voice coil comprises a plurality of turns of fine wire wound on a bobbin. When electrical signals from an audio amplifier are applied to the voice coil, the magnetic field from the internal permanent magnet interacts with the current in the coil windings, thereby causing a force to be applied to the voice coil/bobbin assembly. Since the speaker magnet is fixed in the frame, the voice coil/bobbin assembly moves due to this applied force. As a result, the cone vibrates at a given amplitude and frequency in proportion to the applied current.
In the presence of a large external magnetic field, a conventional loudspeaker will not function properly. The external field interacts with the permanent magnet as well as with the magnetizable material usually employed in construction of the speaker frame and magnet support structure. As a result, the net magnetic field at the voice coil will be inconsistent with normal operating and design parameters. An external field can alter both the magnitude and direction of the net field at the voice coil windings, thus contributing to ineffective cone vibrations in response to electrical signals from the amplifier. Furthermore, if the speaker is removed from the external field, permanent damage to the speaker magnet and support structure can result.
One remedy is to construct the speaker entirely of non-magnetic components which are unaffected by the presence of a strong external magnetic field. In U.S. Pat. No. 4,933,981, issued to Lederer, an electromagnetically shielded sound system adapted for operation in conjunction with a magnetic resonance imaging apparatus is disclosed, which employs separate high and low frequency piezoelectric crystalline transducers which mechanically vibrate in response to applied electrical signals, thereby producing audible sounds which are communicated to a patient by a pair of elongated pneumatic wave guide tubes. Another example of a speaker utilizing piezoelectric transducers is taught in U.S. Pat. No. 4,190,784, issued to Massa.
Strong external magnetic fields also pose safety concerns when placing objects composed of magnetic material in close proximity. Magnetic material experiences a rotational torque proportional to the local magnetic field strength and a linear force of attraction proportional to the local magnetic field gradient. Therefore, objects containing large amounts of magnetic material-can rotate to align themselves with the field and be projected like missiles toward the strongest portion thereof. To counter this undesirable tendency, it is necessary to rigidly fix magnetic objects in place, often with inefficient structures which add weight and complexity.
One application where loudspeakers are operated in the presence of large external fields is in magnetic resonance ("MR") examinations. Since the patient is required to lay down within a large bore in the device for often extended periods of time, it is desirable to communicate with and/or provide sound to such a patient. During an MR examination, a strong magnetic field is generated and varying RF fields are reradiated and processed by a microprocessor. Since the magnetic field distribution must be uniform to a few parts per million over the imaging volume, objects located around the magnet in the fringe magnetic field, such as a conventional loudspeaker containing a large amount of magnetic material, can degrade the uniformity of the external field and cause inaccuracies in system operation.
In the past, communication and music have been provided to patients undergoing MR by using either magnetic transducers positioned outside the fringe field of the magnet such as that taught in U.S. Pat. No. 4,701,952, issued to Taylor, or non-magnetic piezoelectric transducers located near the magnet as in the Lederer apparatus. These prior art systems have shortcomings. The remote speaker location requires that the generated sounds be typically delivered to the patient using non-magnetic wave guide tubing. However, since the sound quality is best when the length of the tubing is short, it is desirable to locate the transducer near the magnet. Since conventional speakers are rendered inoperable in such an environment, and because magnetically inert transducers do not provide uniform frequency response, it is usually necessary to employ multiple transducers in combination with expensive frequency equalizing electronics. Moreover, such transducers are not readily usable with commercial audio amplifiers, necessitating the use of expensive, custom components.
During MR examinations, loud sounds are produced by activation of the gradient fields produced by the MR apparatus. The sound intensity can be very high depending on the type of examination, and the field strength of the MR system. In the past, noise cancellation techniques have been employed to reduce or remove such sounds, an example of which is taught in U.S. Pat. No. 4,696,030, issued to Engozi. These techniques have typically utilized conventional loudspeakers displaced far from the MR magnets to generate sound waves 180 degrees out of phase with the noise. These out-of-phase sounds are combined with gradient sounds when delivered to the patient through wave guide tubes attached to headphones. Consequently, the patient hears sounds which, through phase cancellation, are of much lower intensity then the gradient sounds the patient would otherwise hear during examination. If an efficient loudspeaker were available which could operate within a strong magnetic field, in addition to providing the benefits of communication and music, it could be suitable for use in such noise cancellation systems.